Generation of characteristic fluorescence in a sample solution undergoing analysis requires the solution to be irradiated with a high intensity light beam of selected wavelength, often in the ultraviolet. The source for this beam is typically a monochromator using a high intensity source such as an Xenon arc. However, the high radiation level, which is necessary for the high measurement resolution desired, when it is in the near ultraviolet below 300 nm tends to deteriorate the optical elements in the pre-slit condensing system of the exciting monochromator. In a relatively short time, typically a few hundred hours, the reflectivity of the mirrors drops off drastically, especially in the ultraviolet, with the formation of scattering films on their surfaces where the optical beam strikes. Also, some types of optical filters, if used, tend to solarize or lose their optical UV transmission. These problems further extend to the collimating mirrors and grating of the monochromator. In addition, heat build-up at the entrance slit and in the monochromator may cause wavelength shift and band-width errors.
The interposition of a manually operated shutter between the source and the optical system is a remedy that has been used for many years. However, manually operated shutters are unsuited to modern automatic computer controlled instrumentation which may be called upon to operate unattended for hours.
On the other hand, the obvious idea of including commands in the computer programming routines for opening and closing the shutter at specific times calls for an awkward and often impractical degree of program complexity. Many operating procedures such as establishment of photometric base level are similar to measurement procedures but require the shutter to be locked open or closed. Our invention improves the programming sequence by separating the shutter control from the user selected operating routines thereby providing for opening the shutter only when a signal of activity is received from the instrument's photometer circuit and for closing at a fixed time interval thereafter. This time interval is selected to be long enough to span the usual period of a photometric measurement.
Therefore it is an objective of the invention to provide a protective shutter to isolate the light source of a fluorescent spectrophotometer for optics which could be damaged by excessive extended irradiation.
It is a further object to automatically operate said shutter to open when the instrument activity calls for light from said source.
It is another object to have said shutter automatically close after sufficient time to allow normal activity procedures to be completed.
As a further object complication of instrument routines is avoided by separating the control routine of shutter open period from timing programs of the instrument measuring system.
A further object is to maintain the shutter open if the instrument measurement activity is still continuing when the shutter would normally close.
These objects, as well as provision for locking the shutter open or closed at operator command, are to be accomplished with a minimum program load on the computer so that computer time is not lost from other routines.